Stephanie (Jodie Sweetin) lied about her relationship with Michelle (Mary-Kate and Ashley Olsen) in Full House as she reminisced about her sister in Fuller House. While the youngest Tanner sister never appeared in the Netflix spin-off, she was referenced fairly often, especially during the show's first years. Oftentimes, it would be a casual name-drop just to establish she's still part of the family despite not showing up, like how Stephanie recalled their time together when they were kids. That said, the middle Tanner child may have gotten some of the details of her story wrong.
All core cast members of Full House returned for Fuller House except for the Olsen twins. While Netflix and the people involved in the project attempted to cajole them for a few years, Mary-Kate and Ashley were set on not returning to their acting roots as they've fully moved on from it. This forced the sequel series to work their way around her absence — coming up with the explanation that like the sisters in real life, Michelle is now based in New York City where she manages her fashion empire. Still, the sequel put a considerable effort into incorporating her in storylines, particularly in the first few episodes.
One such moment occurs in Fuller House season 1, episode 2 titled "Moving Day." On the heels of Stephanie and Kimmy's (Andrea Barber) decision to move into the Tanner family home, it took a bit of time for DJ (Candace Cameron-Bure) to learn how to delegate household chores and responsibilities to her sister and best friend. The eldest sister particularly struggled with leaving her kids in Stephanie and Kimmy's care knowing that both have their own quirks, not to mention a contentious relationship while they growing up. While Jackson (Michael Campion) and Max (Elias Harger) were old enough to fend for themselves, Kimmy and Stephanie had to be focused on Tommy — who was essentially an infant. At one point, they needed to change his diapers which had Stephanie saying that she hadn't done it since Michelle was a baby. Looking back at the events of Full House, however, this was never the case.
Full House never showed Stephanie changing Michelle's diapers. In the few instances that it was a plot point in the classic family sitcom, Jesse (John Stamos) and Joey (Dave Coulier) were always the ones who were up to the task. While there's a possibility that Stephanie was recalling something that wasn't shown on the show, the chances of this happening are very slim to none given her age at that time. When Full House began in 1986, Michelle was less than a year old, while Stephanie was five years old — too young to be doing diaper duties for her baby sister. By season 2, Danny (Bob Saget) had already started to potty train Michelle as tackled in episode 2 titled "Jingle Hell." At this point, the youngest Tanner sister was around two years old already while Stephanie was six — still a bit too young to be changing diapers for her sister herself. If anything, it would've been more possible for DJ to do this task from time to time.
This doesn't mean, however, that Stephanie wasn't a good sister to Michelle. While the pair had their disagreements — including fighting over Mr. Bear — they cared for each other. It's just that timeline-wise, Stephanie's claim in Fuller House about changing her sister's diapers in Full House doesn't sound right.